1. Field of the Invention
The present invention relates to a Passive Optical Network (PON) using Wavelength Division Multiplexing (WDM).
2. Description of the Related Art
Most broadcast and communication service subscribers currently use data communication services such as Internet at rates of about several tens of Kbps (bits per second) to several tens of Mbps. These rates are achieved through xDSL, a cable modem, a dial-up modem, metro-Ethernet, etc. Subscribers also use broadcasting services such as cable broadcasting through a coaxial cable, satellite broadcasting through a satellite, terrestrial broadcasting, etc.
Data rates of about 100 Mbps are required to effectively provide subscribers with services such as high-capacity image information, VoD (Video on Demand), and high-quality digital broadcasts. However, the prior art has many limitations in accommodating such data in terms of its capacity and speed. Accordingly, there is a need for implementing an optical network using optical communication. To this end, PON technology has been proposed and developed as an economic way to implement an optical network.
The PON is classified mainly into an Asynchronous Transfer Mode (ATM)-PON, an Ethernet-PON, and a WDM-PON according to its implementation methods.
The ATM-PON and Ethernet PON have a number of limitations including (1) no transparency regarding the speed of data to be provided to subscribers, and (2) complicated Media Access Control (MAC) technologies are required for an Optical Line Terminal (OLT) because the same wavelength is used in the upstream transmission. The ATM-PON and Ethernet PON also have a problem in that it is difficult to accommodate digital broadcasts.
The WDM-PON does not have the limitation on the speed of data, and also does not require a complicated MAC technology. In addition, it is easy for the WDM-PON to accommodate digital broadcasting services.
FIG. 1 shows one prior-art example of a WDM-PON that integrates broadcast and communication services. As shown, the WDM-PON includes an Optical Line Terminal (OLT) 11, Optical Network Terminal/Optical Network Unit (ONT/ONUs) 12, an optical splitter 15, a photoelectric (or optical-to-electrical) converter (O/E) 16, a Radio Frequency (RF) splitter 17, a WDM demultiplexer 13, and a WDM multiplexer 14. The OLT 11 is connected to a broadcasting network for broadcasting services and an Internet Protocol (IP) network for communication services. The OLT 11 receives broadcast signals from the broadcasting network and communication signals from the IP network. The OLT 11 transmits the received signals as optical signals after their photoelectric conversion. In addition, the OLT 11 transfers communication signals, received from service subscribers, to the IP network. The ONT/ONUs 12 are devices on the user side for transferring the broadcast and communication signals, received from the OLT 11, to service users and transferring communication signals from the service users to the OLT 11. The optical splitter 15 splits the broadcast and communication signals from the OLT 11 into a broadcast signal λA and communication signals λ1 to λ32, and transmits them. The photoelectric converter 16 photoelectrically converts the broadcast signal λA split through the optical splitter 15. The RF splitter 17 distributes the converted electric broadcast signal to each of the ONT/ONUs 12. The WDM demultiplexer 13 WDM-demultiplexes the communication signals λ1 to λ32, and transfers them to the ONT/ONUs 12. The WDM multiplexer 14 WDM-multiplexes the communication signals λ1 to λ32 from the ONT/ONUs 12, and transfers them to the OLT 11. λ1 to λ32 are used as wavelength channels for communication, and λA is used as wavelength channels for broadcasting.
Operationally, a broadcast signal λA in downstream transmission is optically split through the optical splitter 15 in the front end of the WDM demultiplexer 13. Then it is photoelectrically converted in the photoelectric converter (O/E) 16. After the converted signal is separated into broadcast signals corresponding respectively to service users through the RF splitter 17, they are transmitted to the respective service users through a coaxial cable.
After being demultiplexed in the wavelength demultiplexer 13 on a wavelength-by-wavelength basis, the communication signals λ1 to λ32 are transmitted to the respective service users.
Disadvantageously, in such a prior art system, the transmission of broadcast and communication signals are not integrated in one wavelength. Instead, the broadcast signal is transmitted with its analog and digital broadcast signals integrated in one wavelength λA. The communication signals are provided to respective subscribers through wavelengths λ1 to λ32 
In addition, in such a prior art system, the optically-split broadcast signal λA is photoelectrically converted. Its analog and digital broadcast signals are transmitted through a cable network. Such a cable network is installed to provide the existing broadcasting services defined in bands up to around 550 MHz. However, it is impossible to accommodate analog/digital broadcasts using the existing cable network because the domestic digital cable broadcasting standard was established to have a band of around 850 MHz for analog and digital broadcast signals.
Thus, in order to accommodate analog/digital broadcasts, such a system (1) requires the lead-in coaxial cable in each subscriber to be replaced with a coaxial cable of an extended band, or (2) to install a new extension coaxial cable with the existing coaxial cable being retained.
FIG. 2 shows another prior-art example of a WDM-PON that integrates broadcast and communication services. As show, the WDM-PON includes an OLT 21, ONT/ONUs 22, a WDM demultiplexer 23, and a WDM multiplexer 24. The OLT 21 is connected to a broadcasting network for broadcasting services and an IP network for communication services. The OLT 21 receives broadcast signals from the broadcasting network and communication signals from the IP network, and transmits the received signals as optical signals after their photoelectric conversion. The OLT 21 further transfers communication signals, received from service subscribers, to the IP network. The ONT/ONUs 22 are devices on the user side for transferring the broadcast and communication signals, received from the OLT 21, to service users, and transferring communication signals from the service users to the OLT 21. The WDM demultiplexer 23 WDM-demultiplexes both the communication signals λ1 to λ32 and the broadcast signals λ33 to λ64, and transfers them to the ONT/ONUs 22. The WDM multiplexer 24 WDM-multiplexes the communication signals λ1 to λ32 from the ONT/ONUs 22, and transfers them to the OLT 21.
In such a prior art system, it is possible to provide high-capacity data communication and broadcasting services because wavelengths for broadcast and communication are given individually to each of subscribers. However, as with the prior art system of FIG. 1, this system does not provide for integrated transmission of the broadcast and communication signals. In addition, this system must include twice as many optical transmitters as the prior art system of FIG. 1. In addition, each subscriber must use two optical receivers in order to receive both a communication wavelength signal and a broadcast wavelength signal. This causes an increase in the cost of the WDM-PON system, consequently imposing a burden on the subscribers. Further, in the WDM-PON, the number of subscribers coincides with the number of wavelengths. Thus, the prior art system of FIG. 2 has a further limitation in that it can handle only half the number of subscribers as compared to the prior art system of FIG. 1, because each subscriber uses two wavelengths.